1. Field of the Invention
This invention relates to timing devices installed in motor driven vehicles such as automobiles, trucks, motorcycles, recreational vehicles, etc., to indicate the elapsed running time of the engine, and more particularly it relates to protective circuits in such devices which make them more reliable in the presence of transient voltage surges that may be encountered in motor vehicle electrical systems.
2. Description of the Prior Art
A preferred method of measuring the actual running time of a motor vehicle engine is disclosed in U.S. Pat. No. 3,355,731. Patentee therein discloses a coulometric timing apparatus employing an electrolytic cell having a sensing wire which is subjected to electrochemical action each time the electrical system of the vehicle engine to which it is attached is energized. At a predetermined time the wire is consumed and the associated circuitry turns on an indicator showing the time has elapsed. Improved capsules are disclosed in U.S. Pat. Nos. 3,711,751, 3,769,557, and 3,944,894. U.S. Pat. No. 3,291,109 is representative of a transistorized sparking system for direct drive of a high voltage ignition transformer. U.S. Pat. No. 3,686,484 discloses a prior art engine cycle counter employing electronic detection means and U.S. Pat. No. 3,875,505 discloses a regulated power supply carried in a motor vehicle instrumentation system to measure various engine parameters such as the number of times the crankshaft has rotated.
Until now there has been no recognition of the problem of momentary high voltage transient surges which can occur in an automotive ignition system that would be of such a magnitude that would destroy the transistor in a coulometric timing circuit.